Ammoniacal leaching of copper ores and concentrates has a long history. Leaching of native copper and of oxidized copper ores (copper oxides, copper carbonates, copper silicates, etc.) presents no substantial difficulty, for such materials react readily with ammoniacal leaching reagents when the leaching operation is carried out with free access of air. Ammonia leaching procedures have been employed since at least the early part of the twentieth century to recover copper from native and oxidized copper ores.
Sulfide copper, nickel and zinc minerals are also known to be soluble in ammoniacal solutions under oxidizing conditions. However, they are not as readily soluble as the oxidized copper minerals or native copper, and there are substantial differences in the ease with which different common sulfide minerals will dissolve. For example, chalcocite, Cu.sub.2 S, dissolves with least difficulty, covellite, CuS, and bornite, Cu.sub.5 FeS.sub.4, dissolve quite readily, and chalcopyrite, CuFeS.sub.2, pentlandite, (Ni,Fe).sub.9 S.sub.8, enargite, Cu.sub.3 AsS.sub.4, and sphalerite, ZnS, dissolve only with considerable difficulty.
Proposals to recover copper from sulfide minerals by ammoniacal leaching go back as far as the late nineteenth century. For example, the Hirsching U.S. Pat. No. 581,160, granted Apr. 20, 1897, proposes to extract copper from copper sulfides by leaching with a solution of aqueous ammonia and a nitrate such as ammonium nitrate. None of these early procedures for direct leaching of copper sulfides ever proved practical, however, and most of the early workers in ammoniacal leaching turned to roasting the sulfides in air and then leaching the oxidized copper from the roasted product, as described for example in the Burghardt et al. U.S. Pat. No. 385,345, of June 29, 1897, and in the Jumau U.S. Pat. No. 883,961 of Apr. 7, 1908. Such procedure did not compete economically with conventional smelting techniques, and had no advantages otherwise to recommend it, and so found no practical utilization.
A few decades ago interest developed in a procedure for extracting nickel from its sulfide ores by treatment with an ammoniacal solution under oxidizing conditions at high temperature and pressure. Copper is frequently associated with nickel in such ores and the procedure was applied to the recovery of copper as well. Such procedure, as described for example in the Forward U.S. Pat. Nos. 2,576,314 granted Nov. 27, 1951, 2,726,934, granted Dec. 13, 1955, and 2,822,263 granted Feb. 4, 1958, involves preparing a slurry of the ground mineral in an aqueous ammonia solution and heating such slurry with active agitation in an autoclave for a period of several hours. Air and ammonia are fed into the autoclave, as required to supply the needs of the leaching reactions. Although the patents refer to conducting such a leaching operation at atmospheric pressure and at a temperature as low as 100.degree. F., they recognize that under such conditions leaching proceeds much too slowly to be of commercial interest. Accordingly, they describe precedures involving temperatures approaching or above the boiling point (typically 175.degree. F. to 225.degree. F.) and pressures of 100 psi. or more.
Relatively little consideration has been given to the use of ammoniacal leaching for the recovery of zinc from its sulfide ores, although the facts that zinc forms complexes with ammonia and that zinc sulfide dissolve in ammoniacal solutions in the presence of suitable oxidants have long been known.
The procedures of the Forward patents are the only ammoniacal leaching procedures that have ever been found suitable for commercial use in the extraction of metal values from sulfide minerals. Such use, however, has been limited to treatment of nickeliferous minerals, including copper-bearing nickel ores and concentrates, at high pressures in autoclaves; and the high value of the nickel recovered has been necessary to support the relatively costly autoclaving operation. No successful commercial use of the process in the treatment of conventional copper or zinc ores or concentrates, in which copper or zinc is the primary or sole metal value, is known.